The present invention relates to a toroidal type continuously variable transmission and, more particularly, to an improvement of a toroidal type continuously variable transmission for a vehicle such as an automobile.
Conventionally, gear type variable transmissions have been used most frequently as vehicle variable transmissions. As gear steels for forming gears, low-alloy steels such as SCr420 and SCM420 are used among other machine structural steels and alloy steels defined by JIS G4051 to G4202. Such machine structural steels as materials are formed into the shapes of gears and subjected to a surface hardening treatment such as cementation or nitriding. However, conventional gear type (automatic) step variable transmissions are discontinuously variable transmission mechanisms. Therefore, a loss is produced during the transmission of power, or a shift shock is generated.
On the other hand, continuously variable transmissions produce no intermittent shift shocks. Accordingly, continuously variable transmissions are superior to gear type step variable transmissions in power transmission characteristics and have high fuel consumption efficiency. For this reason, various researches have been made recently to incorporate continuously variable transmissions in actual automobiles, and belt type continuously variable transmissions are put to use in some automobiles.
One of these continuously variable transmissions is a toroidal type continuously variable transmission including input and output disks and a power roller bearing. This toroidal type continuously variable transmission can transmit higher torque than a belt type continuously variable transmission and hence is considered to be effective as a continuously variable transmission for medium- and large-sized automobiles. Therefore, the development of a high-durability material which can transmit high torque and does not break even at high temperatures is being sought.
Conventional high-durability materials for this toroidal type continuously variable transmission are as follows. That is, as disclosed in Jpn. Pat. Appln. KOKAI Publication No. 7-208568, rolling elements of a power roller bearing as a toroidal type continuously variable transmission component are made of medium or high carbon steel and subjected to carbonitriding, hardening, and tempering. Also, as described in Jpn. Pat. Appln. KOKAI Publication No. 9-79336, machine structural steel containing Cr is used as the material of rolling elements of a toroidal type continuously variable transmission, and the rolling elements are carbonitrided to meet the following conditions. That is, the N amount in the rolling element is 0.2 to 0.6 wt %. At depth d.ltoreq.0.2 Zst where Zst is the depth at which the maximum shearing stress is produced inside the rolling element due to surface contact, the C+N amount is 0.9 to 1.3 wt %, the residual austenite amount is 20 to 45 vol %, and the hardness is Hv500 or more. Additionally, at a depth satisfying 0.5 Zst.ltoreq.d.ltoreq.1.4 Zst, the C+N amount is 0.6 wt %.ltoreq.C+N.ltoreq.1.2 wt %, and the hardness is Hv700 or more.
When a conventional toroidal type continuously variable transmission is driven, a high contact pressure is produced between the input and output disks and the power roller bearing (i.e., on the traction surface of the power roller). Consequently, a high thrust load acts on the power roller bearing, so a rolling contact load similar to that of a roller bearing acts on the bearing. These contact pressure and thrust load produce a high load which is not produced in common rolling bearings. In particular, the traction surface or bearing surface of the power roller readily peels or breaks. This makes the rolling life of power roller bearing surface impossible to prolong. For example, in a toroidal CVT, the contact surface pressure of a traction power transmitter at the maximum torque and minimum speed is Pmax=3.9 GPa (when contact-ellipse major-axis radius a=5 mm and contact-ellipse minor-axis radius b=1.3 mm, maximum dynamic shearing stress generation position; Zo=0.48 b, and maximum static shearing stress generation position; Zst=0.72 b).
Compared to common rolling bearings, a toroidal type continuously variable transmission has its characteristic and serious problem; since the backup stiffness is low unlike in a bearing, repeated bending stress is applied to the power roller, input disk, and output disk to produce high tensile stress (it is found from FEM calculations and results of measurements using a strain gauge that a tensile stress of approximately 90 kgf/mm.sup.2 is produced on the traction surface at the maximum load and minimum speed), so cracks are easily formed from these portions as start points. This makes the fatigue crack resistance impossible to increase (FIGS. 3 and 4). As a series of researches on these problems, rolling life under bending stress is reported (Manuscripts for Japan Tribology Conference, Morioka, 1992-10, pp. 793 to 796). This reference describes that the life is significantly shortened when rolling contact stress and bending stress are combined.
As shown in FIGS. 3 and 4, therefore, the combination of large repeated shearing stress and large repeated bending stress acts on the power roller bearing of this toroidal type continuously variable transmission, resulting in a severe stress loaded state unlike in general-purpose rolling bearings. For example, as shown in FIG. 5, the maximum stress generation position becomes deeper from a conventional peak value P1 to a value P2. Accordingly, simply performing cementation which is considered to be effective to improve the peeling resistance of general-purpose rolling bearings is insufficient to prolong the life of bearings.
In a toroidal type continuously variable transmission, unlike general-purpose rolling bearings, heat is generated when large traction power is transmitted by the input and output disks and the power roller traction surface. The temperature of the contact point is expected to be higher than 200.degree. C., so any conventional bearing material cannot be used. Hence, the amount of alloy element Mo which maintains its hardness even at high temperatures or the amount of alloy element Si which delays readily occurring tissue change are specified.
In Jpn. Pat. Appln. KOKAI Publication No. 9-79336 described above, carbonitriding is performed to set the N amount in the rolling element to 0.2 to 0.6 wt %. At depth d.ltoreq.0.2 Zst where Zst is the depth at which the maximum shearing stress is produced inside the rolling element due to surface contact, the C+N amount is 0.9 to 1.3 wt %, the residual austenite amount is 20 to 45 vol %, and the Vickers hardness is Hv500 or more. Additionally, at a depth satisfying 0.5 Zst.ltoreq.d.ltoreq.1.4 Zst, the C+N amount is 0.6 wt %.ltoreq.C+N.ltoreq.1.2 wt %, and the hardness is Hv700 or more. As indicated by Comparative Example 1 in FIG. 6, these specified values are considered to be effective only to the contact stress. That is, since the hardness near the surface is as low as Hv500, the specified hardness distribution is unsatisfactory for the disks to which the bending stress is further applied. Also, as indicated by Comparative Example 2 in FIG. 6, the depth of 0.5 Zst to 1.4 Zst at which the hardness is specified to Hv700 is set by taking only the rolling contact stress into consideration. Therefore, if the bending stress is combined, this specified hardness is insufficient. Furthermore, although the wear resistance improves when the surface N amount is 0.2 to 0.6 wt %, this surface N amount is too large and significantly deteriorates the processability. Note that the value of Vickers hardness Hv is approximately three times the value of yield stress .delta..sub.y and approximately six times the value of shearing stress .tau..
In Jpn. Pat. Appln. KOKAI Publication No. 7-208568, the rolling elements of the power roller bearing as one component of the toroidal type continuously variable transmission are made of medium or high carbon steel and subjected to carbonitriding, hardening, and tempering. The present invention further improves a material having sufficient durability even under recent severe high-torque conditions.